Priority is claimed to Japanese Patent Application Number JP2005-345504 filed on Nov. 30, 2006, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, which reduce a collector resistance of a vertical PNP transistor used as a power semiconductor element.
2. Description of the Related Art
As an embodiment of a method of manufacturing a conventional semiconductor device, a method of manufacturing the following vertical PNP transistor has been known. A P type semiconductor substrate is prepared, and two N type epitaxial layers are formed on the substrate. An N type buried diffusion layer is formed so as to extend in the P type semiconductor substrate and a first epitaxial layer, and a P type buried diffusion layer is formed so as to extend in the first and second epitaxial layers. Then, a silicon nitride film is selectively formed over the P type buried diffusion layer in the surface of the second epitaxial layer. Thereafter, the P type buried diffusion layer is thermally diffused. By performing the thermal diffusion with the silicon nitride film formed, the upward expansion of the P type buried diffusion layer under the silicon nitride film is checked. Moreover, in a region over which a silicon nitride film is not formed, the P type buried diffusion layer expands upward and a concave portion is formed in the upper surface of the P type buried diffusion layer. On the other hand, in the surface of the second epitaxial layer, a LOCOS oxide film is formed in a region other than that where the silicon nitride film is formed. Subsequently, by removing the LOCOS oxide film, concave and convex portions are formed in the surface of the second epitaxial layer. Next, a collector region is formed by utilizing the concave portion of the second epitaxial layer and the upward expansion of the P type buried diffusion layer. Furthermore, a base region and an emitter region are formed by utilizing the convex portion of the second epitaxial layer and the concave portion of the P type buried diffusion layer. With the above-described method, breakdown voltage (VCEO) between the emitter and the collector of the vertical PNP transistor is increased, and thus the vertical PNP transistor, which decreases saturation voltage (Vce), can be formed (This technology is described for instance in Japanese Patent Application Publication No. 2000-232111 (pp. 3-4, FIGS. 1 to 3), for example.)
As described above, in a conventional semiconductor device, for example, a power vertical PNP transistor and a control NPN transistor are monolithically formed. In the vertical PNP transistor, it is necessary to make an N type epitaxial layer thicker in order to improve breakdown voltage characteristics. On the other hand, in the control NPN transistor, there is the following problem. When the film thickness of the epitaxial layer is increased, an isolation region is extended in the lateral direction, and thus it is difficult to reduce a device size. That is, the monolithic formation of the power vertical PNP transistor and the control NPN transistor causes a problem in which the breakdown voltage characteristics of the power vertical PNP transistor and the reduction in the device size of the control NPN transistor are in a trade-off relationship.
Moreover, in a method of manufacturing a conventional semiconductor device, two epitaxial layers are formed on the P type semiconductor substrate. The P type buried diffusion layer to be used as the collector is formed so as to extend in the first and second epitaxial layers. The upward expansion of the P type buried diffusion layer is partially checked by the forming region of the silicon nitride film which is formed on the surface of the second epitaxial layers. However, even for the P type buried diffusion layer which is located under the silicon nitride film, a wraparound by oxygen, or the like, causes a problem that it is difficult to partially check the upward expansion. To be more precise, in a region where the upward expansion of the P type buried diffusion layer is to be checked, the upward expansion can be confined to only about 0.5 (μm) above the region. Accordingly, in order to satisfy desired breakdown voltage characteristics, it is necessary to form epitaxial layers of a two-layer structure, thus causing a problem that the manufacturing costs are increased.
In addition, in a method of manufacturing a conventional semiconductor device, the LOCOS oxide film is formed in the surface of the second epitaxial layer and then removed to form concave and convex portions on the surface of the epitaxial layer. Subsequently, the P type buried diffusion layer to be used as the collector region is formed from the surface of the epitaxial layer in which the concave portion is formed, and thus the P type diffusion layer is connected to the region of the P type buried diffusion layer, which expands upward. However, in some cases, the silicon nitride film is not formed in a desired region over the P type buried diffusion layer due to the displacement of a mask or the like which occurs when the silicon nitride film formed on the epitaxial layer is selectively removed. In such a case, a region where the P type diffusion layer overlaps the P type buried diffusion layer is reduced, and thus a collector resistance cannot be reduced. That is, the following problem is caused. Since element characteristics vary due to the displacement of a mask in each process, a highly accurate positioning is required. Accordingly, a manufacturing process is complicated.
Moreover, in a method of manufacturing a conventional semiconductor device, for the purpose of forming epitaxial layers on a semiconductor substrate, for example, a vapor phase epitaxial growth device employing a vertical reactor is used. When forming epitaxial layers, there is a problem that boron (B), which is a P type impurity diffused over the semiconductor substrate, tends to be easily auto-doped. In a case where an introduced amount of an impurity is increased in order to reduce a sheet resistance value of a buried diffusion layer, the amount of autodoping increases.